1. Field of the Invention
The present invention relates generally to a displacement detector, and more particularly to a displacement detector for detecting the displacement of an object by detecting a magnetic field which varies as a movable member moves with the displacement of the object.
2. Description of the Prior Art
One conventional displacement detector for detecting the displacement (which may include rotary motion) of an object by detecting a magnetic field which varies as a function of the movement of a movable member caused by the displacement of the object, is basically constructed as shown in FIG. 6(a) of the accompanying drawings.
The displacement detector shown in FIG. 6(a) comprises a bobbin 20 made of an insulating material, a movable member 200 of a magnetic material disposed in the bobbin 20 and vertically movable as a function of the displacement of an object (not shown), a primary coil 210 connected to a signal generator 10 and wound around the bobbin 20, and two secondary coils 220a, 220b connected respectively to rectifier circuits 30, 40 and wound around the bobbin 20 at positions spaced in the direction in which the movable member 200 is movable in the bobbin 20.
The rectifier circuits 30, 40 respectively have rectifying diodes 310, 410 connected in series with the secondary coils 220a, 220b, respectively, smoothing capacitors 320, 420 connected in parallel to the secondary coils 220a, 220b, respectively, resistors 330, 430 connected in parallel to the secondary coils 220a, 220b, respectively, and output terminals 80, 90. The secondary coils 220a, 220b have negative terminals 30b, 40b coupled in common to a ground potential 100.
Operation of the displacement detector will be described with reference to FIG. 6(b).
It is assumed that the secondary coils 220a, 220i b are wound in the same direction. Under a magnetic field generated by the primary coil 210 energized by an AC signal applied by the signal generator 10, the movable member 200 is moved toward one of the secondary coils, e.g., the coil 220a, and the other secondary coil 220b produces a counter electromotive force tending to prevent such movement of the movable member 200. At this time, currents flow in opposite directions through the secondary coils 220a, 220b. Assuming that the upward movement (in FIG. 6(a)) of the movable member 200 is positive (+) displacement and the downward movement thereof is negative (-) displacement, the output terminals 80, 90 produce respective output signals a, b as shown in FIG. 6(b) in response to the displacement D of the movable member 200. More specifically, when the movable member 200 is displaced in the positive (+) direction, the rectified output a is increased, whereas the rectified output b is reduced. Conversely, when the movable member 200 is displaced in the negative (-) direction, the rectified output a is reduced, whereas the rectified output b is increased. Therefore, the absolute value of the difference between the rectified outputs a, b indicates the magnitude of the displacement of the movable member 200, and the sign of the difference indicates the direction of the displacement. Half of the sum of the rectified outputs a, b. i.e., (a+b)/2, is of a constant value A1 at all times, and coincides with the cross-over point of the curves a, b.
For obtaining a large detected signal from the displacement detector shown in FIG. 6(a), it is preferable in general that the difference (a-b) between the rectified outputs be large, and that the average value of the output voltage (corresponding to the voltage at the point A1 where the displacement is 0) be of a certain small value so as to allow easy connection to a control circuit at a next stage. Where the difference (a-b) between the rectified outputs is large, the S/N ratio is high and highly reliable signal control is made possible. If the average value A1 of the output voltage is about 2.5V, then the output voltage of the differential displacement detector can directly be applied to a control circuit such as a microcomputer at a next stage, without requiring any transformation therebetween.
U.S. Pat. No. 4,437,531, for example, discloses a modified design of the above displacement detector, wherein the angular displacement of an input shaft (object with its displacement to be detected) with respect to an output shaft of an automotive power steering system is detected by detecting a magnetic field which varies as a movable member is moved in response to the displacement (rotation) of the intput shaft. In this modified displacement detector, the rectified outputs from two secondary coils are applied to a differential amplifier, which amplifies the difference between the applied outputs and supplies the same to a next control circuit.
If the conventional displacement detector such as shown in FIG. 6(a) is used in an automotive power steering device and an amplifier is employed to increase the difference (a-b) between the rectified outputs as disclosed in U.S. Pat. No. 4,437,531, then the overall arrangement is complex, operation reliability thereof is lowered, and the cost is increased, because of an increased number of components. One way of increasing the difference (a-b) between the rectified outputs would be to increase the stroke that the movable member 200 can move. However, this would not be practical since the required space would be increased and the structural rigidity would be lowered. Alternatively, the difference (a-b) between the rectified outputs could be increased by increasing the number of turns of each of the coils 220a, 220b. With this alternative, the rectified output difference would be increased as indicated by curves c, d in FIG. 6(b), but the average output voltage would also be increased to a value A2, and hence the output voltage could not generally be used directly for subsequent signal processing.